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By Michel Rose

PARIS (Reuters) - Inside the election war rooms of Paris, French politicians on both left and right are waking up to the threat Emmanuel Macron poses ahead of next year's presidential election and stepping up attacks on the fresh-faced former economy minister.

Macron quit President Francois Hollande's government last month, pledging to "transform France" and taking the most concrete step so far towards a presidential bid he has not yet made official.

Government and opposition politicians have branded the 38-year-old former M&A banker a "traitor" and "populist light".

With poll after poll showing far-right leader Marine Le Pen assured of getting to the second round but losing the runoff in May to whoever faces her, Socialists and conservatives realize Macron's pitch for the middle ground could cost them the remaining place.

"Macron is a danger for us," a government minister said on condition of anonymity. "He's going to steal votes on the left and the right, although he can't possibly do more than 18 percent and reach the second round himself."

Former prime minister Alain Juppé, the leading candidate in the center-right's primaries, called Macron "Brutus" after his resignation from Hollande's government, and also has reason to fear him, even if his entourage plays down that threat.

"There's a window for him if (former president) Nicolas Sarkozy wins the primaries and Hollande is not a candidate," Juppé's campaign chief Gilles Boyer told Reuters. "But he's prisoner of events beyond his control."

Speaking to Reuters on condition of anonymity, close advisors within the "En Marche!" political movement Macron created last April say the backlash has failed to tarnish his public image.

His popularity jumped four points after his resignation, according to an Odoxa poll published this month, with 45 percent of respondents considering he would make a good president, the second-highest score after Juppé.

In one scenario tested in an Elabe poll that assumes Sarkozy, not Juppé, is the conservative champion, Macron is seen coming third in the first round with 18 percent, one point behind Sarkozy, who would just manage to reach the runoff.

PRACTICAL CHALLENGES

But Macron has very practical challenges to tackle.

With only a dozen permanent staff, people in the movement say they need to hire another half a dozen by mid-October.

Never having been elected, Macron cannot count on public funding for a presidential campaign like candidates backed by established parties, and needs to raise funds which under French law are capped at 7,500 euros per person per year.

Critics also point out the movement's 80,000 members were able to sign up for free, and do not amount to the army of grassroots activists other candidates will mobilize.

Macron nonetheless managed to deploy hundreds of volunteers this summer in 50 cities in a door-to-door campaign meant to collect voters' grievances.

The small team is now preparing a "diagnosis" for the country - struggling with high unemployment and which has endured several deadly Islamist attacks - that Macron said he would unveil in October in three rallies outside Paris, the first in Strasbourg on Oct. 4.

"We want people to think 'that's the first time a guy tells me what I'm suffering from'," a source in his inner circle said.

A "transformation plan" with 6-7 priorities will follow mid-November. Only then will he consider whether to run for president, the advisor said.

DIRTY TRICKS

Meanwhile, Hollande's party officials are working behind the scenes to strongarm lawmakers into not rallying behind the former minister, Macron's advisors said, with Socialist Party chief Jean-Christophe Cambadelis threatening to expel Macron supporters from the party.

Asked about the allegations, a party official said according to party rules members should support the party's official candidate in any election.

Several French and European policymakers canceled their appearance at a gathering of European Social Democrats in Lyon where Macron was invited to speak at the weekend.

Investigative weekly Le Canard Enchaine said Hollande had asked Italian Prime Minister Matteo Renzi to turn down the invitation. The French president's office denied it had intervened and Renzi's office said he was never scheduled to attend the Lyon summit.

Macron's team say if Hollande's ratings, the lowest for a president in France's post-war history, fail to pick up by the end of the year, more politicians will rally behind him.

One recruit to his cause is Daniel Cohn-Bendit, hero of the 1968 student protests, who said Macron was best placed to prevent a Sarkozy-Le Pen runoff next year.

Macron's advisor said finding enough candidates to run in France's 577 parliament constituencies in June would not be a problem either. "En Marche can field candidates everywhere. Our goal is to build something for the long term."

Others are not so sure.

"He's a brilliant guy," junior minister Jean-Vincent Placé said. "But he is deluding himself on who will really support him and where he will be in two or three months."